Abstract Topramezone is a new, highly selective herbicide of pyrazole structure for the post‐emergence control of broadleaf and grass weeds in corn. The biokinetic properties and mode of action of topramezone were investigated in plants of Setaria faberi Herrm, Sorghum bicolor (L.) Moench, Solanum nigrum L. and the crop species corn ( Zea mays L.). Within 2–5 days after treatment, topramezone caused strong photobleaching effects on the shoot, followed by plant death of sensitive weeds. The selectivity of topramezone between corn and the weed species has been quantified as above 1000‐fold. By virtue of the plant symptoms and the reversal of the effects in Lemna paucicostata L. by adding homogentisate, it was hypothesized that topramezone blocks the formation of homogentisate, possibly through inhibition of 4‐hydroxyphenylpyruvate dioxygenase (4‐HPPD). Indeed, topramezone strongly inhibited 4‐HPPD activity in vitro , with I 50 values of 15 and 23 n M for the enzyme isolated from S. faberi and recombinant enzyme of Arabidopsis thaliana L. respectively. The enzyme activity from corn was approximately 10 times less sensitive. After root and foliar application of [ 14 C]topramezone, equivalent to field rates of 75 g ha −1 , the herbicide was rapidly absorbed and systemically translocated in the plant. Only marginal differences between leaf uptake and translocation of topramezone by the weeds and corn were found. Metabolism of foliar‐applied [ 14 C]topramezone was far more rapid in corn than in the weeds. A more rapid metabolism combined with a lower sensitivity of the 4‐HPPD target enzyme contributes to the tolerance of corn to topramezone. Copyright © 2007 Society of Chemical Industry